Which way does the rubber ducky go? The two could be highly related.
Im right handed. If I'm in the tub, I swish the hot water to the back of the tub with more right hand motion than left. This initiates a clockwise motion.
If I shower, the nozzle is aimed more away from the curtain which, in my case, is to my left. This creates clockwise motion, too.
The drain pipes also are factors. Water flow has a non-uniform flow rate through a bend. A bend is, likely, just beneath your tub. This could have some effect on the flow, I think.
We know time flies, we just can't see its wings.